Motor potentials evoked by navigated transcranial magnetic stimulation in healthy subjects.
ABSTRACT Navigated transcranial magnetic stimulation (TMS) is a tool for targeted, noninvasive stimulation of cerebral cortex. Transcranial stimuli can depolarize neurons and evoke measurable effects which are unique in two ways: the effects are caused directly and without a consciousness of the subject, and, the responses from peripheral muscles provide a direct measure for the integrity of the whole motor pathway. The clinical relevance of the method has not always been fully exposed because localizing the optimal stimulation site and determining the optimal stimulation strength have been dependent on time-consuming experimentation and skill. Moreover, in many disorders it has been uncertain, whether the lack of motor responses is the result of true pathophysiological changes or merely because of unoptimal stimulation. We characterized the muscle responses from human primary motor cortex system by navigated TMS to provide normative values for the clinically relevant TMS parameters on 65 healthy volunteers aged 22 to 81 years. We delivered focal TMS pulses on the primary motor area (M1) and recorded muscle responses on thenar and anterior tibial muscles. Motor threshold, latencies and amplitudes of motor-evoked potentials, and silent period duration were measured. The correction of the motor-evoked potential latency for subjects' height is provided. In conclusion, we provide a modified baseline of TMS-related parameters for healthy subjects. Earlier such large-scale baseline material has not been available.
- Electroencephalography and Clinical Neurophysiology 09/1994; 91(2):79-92.
- [show abstract] [hide abstract]
ABSTRACT: Stimulus-response curves for motor evoked potentials (MEPs) induced in a hand muscle by transcranial magnetic stimulation (TMS) were constructed for 42 subjects with the aim of identifying differences related to age and sex. There was no effect of age on the resting threshold to TMS, the maximal amplitude of the MEP that could be evoked (MEP(max)) or the maximal slope of the stimulus-response curve. However, higher stimulus intensities were required to achieve both MEP(max) and the maximal slope in the older subjects. The trial-to-trial variability of MEPs was greater in the older subjects, particularly at intensities near threshold. There was a significant interaction between age, threshold and trial-to-trial variability of MEP amplitude. Overall, MEP variability fell markedly as stimulus intensity increased above threshold but less rapidly in older than in younger subjects. Females tended to have larger MEP variability than males, but age and threshold were much stronger modulators than sex. These differences in input-output characteristics are likely to be due either to a decreased number of spinal motoneurones being activated synchronously in older subjects, or to the activation of the same number of motoneurones in a less synchronous manner, leading to phase cancellation in the surface electromyogram.The Journal of Physiology 02/2003; 546(Pt 2):605-13. · 4.38 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: Previous studies have shown that transcranial magnetic stimulation (TMS) of the sensorimotor cortex can induce a suppression of cutaneous perception from the fingers of the contralateral hand. In this work, 17 normal subjects were submitted to focal TMS of frontal and parietal scalp sites of each hemisphere. TMS was delivered at two interstimulus intervals (20 and 40 ms) following a cutaneous electrical stimulation of the first, third and fifth digits of either hand or both hands near the subjective threshold of perception. The aim of our study was to investigate whether TMS could detect an asymmetrical hemispheric specialization in the sensory perception of unimanual and bimanual, ipsilateral and contralateral sensory stimuli. At each interpulse interval, the right parietal cortex was significantly more sensitive to TMS interference with stimulus detection for both contralateral and ipsilateral stimuli compared with the left parietal cortex. These effects were mainly evident during bimanual discrimination tasks. Our results are indicative of an interhemispheric difference in the detection of cutaneous sensation, showing right hemispheric prevalence in the perception of contralateral as well as of ipsilateral stimuli. They provide neurophysiological support in normal humans to the clinical evidence which indicates that right hemisphere lesions can indeed produce deficits in the perception of ipsilateral sensory stimuli.Brain 10/1999; 122 ( Pt 9):1721-9. · 9.92 Impact Factor